Assessing the Performance of Cobalt-Phthalocyanine Nanoflakes as

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

pubs.acs.org/JPCC

Assessing the Performance of Cobalt Phthalocyanine Nanoflakes as Molecular Catalysts for Li-Promoted Oxalate Formation in Li−CO2− Oxalate Batteries Moein Goodarzi,† Fariba Nazari,*,†,‡ and Francesc Illas*,§ Department of Chemistry and ‡Center of Climate Change and Global Warming, Institute for Advanced Studies in Basic Sciences, Zanjan 45137-66731, Iran § Departament de Ciència de Materials i Química Física & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, C/Martí i Franquès 1, Barcelona 08028, Spain Downloaded via UNIV OF TEXAS AT EL PASO on November 7, 2018 at 00:00:35 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: Removal of O2 molecules from the cathode environment in the Li-based battery has led to introduction of the Li−CO2 battery as the novel and promising source of energy storage. In spite of CO2 capture through the reversible reaction between Li atoms and CO2 molecules at the cathode, the performance of the Li−CO2 battery is hampered by formation of the Li2CO3 insulating product in the discharge process and its difficult decomposition in the charging process. Hereby, we explore the possible improvement of the performance of the Li−CO2 battery through replacement of Li2CO3 by Li2C2O4 as the discharge product. This is achieved by systematic addition of Li and CO2 to a cobalt phthalocyanine (CoPc) nanoflake employed as the molecular catalyst in the cathode of the Li−CO2 battery by means of computational density functional theory-based methods. The present results predict high adsorption energy of the CO2 molecules (−2.16 eV), low Li-intercalation voltage (1.45 V), reveal the important and constructive influence of the electrolyte (dimethyl sulfoxide) on the adsorption and decomposition energies and Li-intercalation voltage, and suggest a through-space electron transfer mechanism for the formation of the Li2C2O4 product on the CoPc nanoflake. Moreover, the high electron affinity of the CoPc nanoflake along with the suitable thermodynamics and kinetics of electron transfer from the CoPc nanoflake to the CO2 molecules during the formation of the Li2C2O4 product confirm the potential abilities of the CoPc nanoflake to be used in the Li−CO2 battery. Therefore, present results provide a sound assessment of the capability of the CoPc nanoflake as a cathode material in the Li−CO2 battery and show that this provides a possible and effective solution to improve the performance of the Li−CO2 battery and to introduce new Li−CO2−oxalate batteries.

1. INTRODUCTION

The lithium-ion batteries introduced initially by Sony included a hard carbon anode and a LiCoO2 cathode (denoted as LCO battery).13 Later on, the hard carbon anode was replaced by graphite, resulting in an improved efficiency but replacement of LiCoO2 by similar materials such as LiFePO4 or LiMn2O4 did not result in a significant improvement.14 Note that current studies in layered-oxide type cathodes show that materials with high energy density similar to that of lithium nickel cobalt aluminum oxide (NCA battery) and lithium nickel cobalt manganese oxide (NCM battery)15 can be successfully used in commercial batteries. Nevertheless, several drawbacks such as low energy storage, low charging, and discharging speed in practical applications remain,16 which call for suitable alternatives to LiCoO2, although this is far from being a simple problem. For instance, Li−S-based batteries face problems derived from Li 2 S2 and Li 2S

1−3

The CO2 electrocatalytic reduction is a promising approach encompassing two fundamental aspects. First, it decreases the CO2 concentration in the Earth atmosphere thus mitigating its concomitant greenhouse gas effect. Secondly, it opens the way to produce useful materials with some possible application in energy storage.4 Note, however, that the latter requests compounds with metal−ligand interactions featuring high electron transfer (ET) ability. In most of the reported cases, metal subunits play a crucial role in ET to CO2. The metal subunits often involve efficient but rare and precious metals such as Re and Pd,5−7 and efforts have been directed to replace these by Earth-abundant transition metals such as Co and Fe.8−11 In this sense, connecting CO2 electrocatalytic reduction and Li can provide a promising approach for the energy storage process in the related batteries. In fact, the high specific energy (energy per mass unit) and high energy density (energy per volume unit) of Li-based batteries have received special attention as the source of energy storage.12 © XXXX American Chemical Society

Received: July 4, 2018 Revised: October 17, 2018 Published: October 22, 2018 A

DOI: 10.1021/acs.jpcc.8b06395 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

and CoPc−Li−CO2 backbones with different Li:CO2 ratios. From the calculated results for the Li-intercalation voltage, a comparative study of the three possible mechanisms for the ET from the CoPc backbone to the CO2 molecules is carried out. In particular, we consider (i) 2CO2 + 2e → [C2O4]− + e → [C2O4]2−; (ii) 2CO2 + 2e → [CO2]− + CO2 + e → [C2O4]2−; and (iii) 2CO2 + 2e → [C2O4]2−. The computed rate constant for these three possible charge transfer pathways sheds light on the necessary conditions for the design of improved materials to be used as cathodes in the Li−CO2−oxalate batteries. We hope that the present computational study will motivate experimental researchers to investigate this suggested solution as a possible improvement of the performance of the Li−CO2 battery.

depositions and the presence of poly-sulfide (Li 2 S n ) intermediates, and both affect the efficiency of this type of batteries.17−27 In a similar way, Li−O2 and Li−[O2 + CO2] batteries exhibit problems arising from the formation of the Li2O2 and Li2CO3 insulation depositions and the presence of active O2− and O22− intermediates.17 Removal of O2 from the cathode environment in Li−[O2 + CO2] batteries resulted in a new type of batteries usually referred to as Li−CO2 batteries,28 with promising energy storage efficiency and ability to capture and release CO2. The final product in these new batteries is produced according to reactions R1−R4.29 2CO2 + 2e− → C2O4 2 −

(R1)

C2O4 2 − → CO2 2 − + CO2

(R2)

CO2 2 − + C2O4 2 − → 2CO32 − + C

(R3)

2Li+ + CO32 − → Li 2CO3

(R4)

2. COMPUTATIONAL DETAILS Atomic structures of the different CoPc systems reported in the present paper have been obtained through full-geometry optimization within DFT. In particular, the PBE0 hybrid functional39 has been chosen and the electron density expanded in a standard Gaussian type orbitals basis of 631G(d) quality. In addition, the D3 method proposed by Grimme et al.40 has been chosen to account for the contribution of dispersion terms not included in the standard functionals. This computational setup will be referred to as PBE0-D3/6-31G(d). Nevertheless, calculated energy values have been further refined by means of single point energy calculations at the PBE0-D3/6-31G(d) optimized geometries using the more complete cc-pVDZ and Def2-TZVP basis sets. A comparison between the two set of values shows that the trend of adsorption energies obtained at the computationally more affordable PBE0-D3/cc-pVDZ level are in good agreement with those arising from the PBE0-D3/Def2-TZVP calculations; see Table S1 in the Supporting Information. Closed shell singlet and doublet potential energy surfaces (PESs) were explored for species with even and odd number of electrons, respectively. Subsequently, we have calculated adsorption energy per Li atom and per CO2 molecule (Eads1 and Eads2) as in 1 and 2.

The environmentally friendly, high thermal stability, and nonoxidative characteristic are the unique features of the oxalate intermediate formed in R1 mentioned above.28,30 However, one of the remaining challenges in Li−CO2 batteries is preventing the formation of the Li2CO3 as the final product in the cathode, which requires stabilizing the oxalate intermediate in step R1 and blocking steps R2−R4.28 This process leads to a new type of Li−CO2 batteries denoted as Li−CO2−oxalate batteries. Chen et al.29 used a Mo2C/carbon nanotube cathode and argued that the Mo2C subunit stabilizes the oxalate intermediate, thus preventing the formation of the Li2CO3 insulating film and resulting in a higher efficiency. Note also that decomposition of the oxalate intermediate to CO2 in the charging process requests much lower energy than that required to decompose the Li2CO3 product.28 From the discussion above, it appears that carbon materials are likely to facilitate and stabilize oxalate formation with concomitant increase in the battery efficiency. Among the possible materials able to fulfill this requirement, metal phthalocyanines (MPcs) are attractive because of various specific features. MPcs have four isoindole units connected through nitrogen atoms along with a central transition metal (M), which can be active centers for the electrochemical reactions involved in these batteries. Moreover, unique properties of MPcs with a broad choice of metal centers31 resulted in many practical applications, such as gas sensors,32 active nanoscale catalysts,33 organic light emitting diodes,34 solar cells,35 and biochemical applications.36,37 Liao and Scheiner38 carried out a systematic study of the properties of several MPc (M = Fe, Co, Ni, Cu, Zn, and Mg) and reported that while there is no significant difference among the ionization potentials of the MPcs, implying similar electron donor ability, there were variations in the electron affinity with CoPc exhibiting the highest value. Among the systems scrutinized, the binding energy of the central M to the Pc is also highest for CoPc.38 The salient properties of CoPc suggest that this compound may constitute a suitable molecular catalyst for the formation of a stable oxalate product in the cathode of the Li−CO2−oxalate batteries. Hence, a theoretical study has been carried out to explore the possible paths of oxalate formation through systematic addition of Li atoms and CO2 molecules to CoPc. By means of state-of-the-art computational methods based on density functional theory (DFT), we provide a comparative study of the CoPc−CO2−Li

Eads1 = [E1 − nE(Li) − E2]/n

(1)

Eads2 = [E1 − mE(CO2 ) − E2] /m

(2)

In eqs 1 and 2, E1 stands for the energy of MPc−(CO2)m− Lin or MPc−Lin−(CO2)m, where MPc denotes a metal phthalocyanine (metal = Fe, Co, and Li) and m and n = 0− 4, and in a similar way, E2 stands for the corresponding energy of the same MPc−(CO2)m−Lin or MPc−Lin(CO2)m species but after elimination of Li atom(s) or CO2 molecule(s). A similar notation is used for the adsorption energy of the compounds with different dopant species of CH4, N2, and H2O. The evaluation of kinetic and thermodynamic properties related to the ET reactions, requests diabatic initial (a) and final (b) states. To obtain the geometry of these diabatic states, constrained DFT (CDFT)41−43 is one of the possible useful tools. The CDFT approach minimizes the DFT energy functional under arbitrary constraint via a Lagrange multiplier technique. Next, the electronic coupling (Hab), the energy difference between the (a) and (b) diabatic states (ΔG°) and the reorganization energy (λ) are computed as detailed in previous work.44 These three parameters are essential to B


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The Journal of Physical Chemistry C estimate the rate constant (kab) in the ET reactions following the Marcus theory45−47 as given in eq 3. ÄÅ É ÅÅ (ΔG° + λ)2 ÑÑÑ 2π 1 2 Å ÑÑ Å kab = Hab expÅÅ− Ñ ÅÅÇ ℏ 4πλkBT 4λkBT ÑÑÑÖ (3) All structural optimizations, single-point energy calculations and estimation of diabatic states have been carried out using the NWChem48 package. Moreover, the multiwfn49 package has been used to provide the electron density used for the calculation of the atoms in molecules (AIM) charges.

3. RESULTS AND DISCUSSION We recall that the main objective of the present work is to explore the possible improvement of the performance of the Li−CO2 battery by replacing the Li2CO3 discharge product in the cathode of the Li−CO2 battery by Li2C2O4. To this end, we make use of a simple yet realistic model, which makes use of the high electron affinity of CoPc species proposed as the cathode material in the present work. Also, because Li+ ions migrate to the cathode through the electrolyte and electrons move through the external electrical circuit, the model makes use of the [CoPc]− and [Li]+ ions, respectively. ET from the cathode material, here represented by the [CoPc]− ion, to the adsorbed CO2 molecules, [C2O4]2− formation along with [Li]+ adsorption and desorption of the Li2C2O4 product are the key modeled steps. An important advantage of this model is that it can reveal details about the ET mechanism not easily accessible by experiments on the Li−CO2 battery. One of the key factors in the MPc synthesis is the binding energy of the central M atom because it is related to its thermodynamic stability. The binding energy of the metal atom in CoPc (−10.49 eV)38 is larger than that of FePc (−9.81 eV),38 LiPc (−7.05 eV), H2Pc (−9.30 eV), NiPc (−9.90 eV),38 CuPc (−6.96 eV),38 ZnPc (−5.66 eV),38 and MgPc (−8.14 eV).38 Moreover, the CoPc electron affinity (−3.19 eV)38 is also larger than that of MPc (M = Fe, Ni, Cu, Zn, and Mg) systems. Therefore, we have selected CoPc to investigate its suitability as a molecular catalyst for the formation of the stable oxalate product in cathodes of the Li−CO2−oxalate batteries. 3.1. Properties of CoPc−CO2, CoPc−Li, CoPc−CO2−Li, and CoPc−Li−CO2 Systems. The cathode materials in the Li−CO2−oxalate batteries are involved in adsorption of CO2 from the atmosphere and of Li from the electrolyte. Therefore, it is convenient to calculate the adsorption energy of the CO2 molecule and of the Li atom on the CoPc backbone. It is necessary to point out that the “Li atom” term is used to emphasize the presence of both Li+ and anionic species (electron carriers) in the cathode of the Li battery. This means that all Li species in the reported structures bear a positive charge of +1. Addition of a CO2 molecule or of a Li atom to the CoPc backbone results in the formation of four CoPc− CO2 and three CoPc−Li systems, respectively, with the most stable ones (CoPc−CO2-I and CoPc−Li1-I) being reported in Figure 1. A linear geometry of the CO2 molecule accompanied with a small adsorption energy of about −0.30 eV and a noncovalent interaction between the CO2 molecule and the CoPc backbone are the most salient features of the CoPc− CO2 system. Larger values, ranging from −0.89 to −1.40 eV, are found for the interaction of Li with the CoPc backbone (Table S2 in the Supporting Information). To assess the role of the central M atom of MPc in the Li and the CO2

Figure 1. Face (left), top (center), and side (right) views of the most stable structures of CoPc−CO2 and CoPc−Li. The I symbol is just to show the most stable species.

adsorption energy, we have calculated adsorption energy values of the CO2 molecule and of the Li atom in the FePc and LiPc backbones and included H2Pc for comparison. The calculated results show that the adsorption of the CO2 molecule is independent of central moiety of the phthalocyanines (Pc). In contrast, the adsorption energy of the Li atom in the CoPc (−1.40 eV) is smaller than that corresponding one for LiPc (−3.60 eV), H2Pc (−2.65 eV), and FePc (−1.87 eV). Because discharge and charging processes involve addition and removing of the Li atom in the backbone, respectively, systems with large values of the adsorption energy are not suitable candidates for the cathode of Li-based batteries. Therefore, based on reported adsorption energies, the CoPc backbone appears to be a better choice than FePc, LiPc, and H2Pc. Addition of the Li atom to the reported structures of the CoPc−CO2 system leads to four local minima on the singlet PES of the CoPc−CO2−Li system. Figure 2 displays the structure of the CoPc−CO2−Li1-I species, which involving direct interaction between the Co atom of the CoPc and the C atom of the CO2 molecule, is the most stable for the CoPc− CO2−Li system. Here, the adsorption energy of the Li atom to CoPc−CO2 leading to the CoPc−CO2−Li1-I species (−1.90

Figure 2. Face (left), top (center), and side (right) views of the most stable structures of CoPc−CO2−Li and CoPc−Li−CO2. The I symbol is just to show the most stable species. C


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3 appears to be the most stable within the singlet PES. The noticeable adsorption energy per CO2 molecule of −0.71 eV is

eV) is larger than the one corresponding to addition of Li atoms to the CoPc, leading to the CoPc−Li1-I species (−1.40 eV). Hence, the presence of the CO2 molecule has a synergic effect on the adsorption of the Li atom. Among the three reported structural geometries for the CoPc−Li−CO2 system on the singlet PES, CoPc−Li1−CO2-I species (Figure 2) exhibits the highest thermodynamic stability. This structure is obtained regardless whether concerted or sequential adsorption of the Li and CO2 molecule or direct adsorption of the CO2−Li complex on the CoPc. The CO2 adsorption energy on the CoPc−Li1-I leading to the CoPc−Li1−CO2-I species is noticeable (−0.75 eV), although accompanied by a linear geometry of the CO2 molecule and noncovalent interactions. The role of the Li atom on enhancing the adsorption energy of the CO2 molecule is clear because it goes from −0.30 eV on the CoPc leading to the CoPc−CO2 system to −0.75 eV on CoPc−Li. In spite of the higher thermodynamic stability of the CoPc−CO2−Li1-I system, we have investigated oxalate formation by starting from both the CoPc−Li1−CO2-I and the CoPc−CO2−Li1-I species. 3.2. Formation of CO2 and/or Li Promoted Oxalate on the CoPc Cathode of the Li-Based Battery. The results show that the CoPc−CO2−Li1-I system is not a suitable candidate for the formation of the C2O4 subunit on the CoPc by increasing concentration of the Li atom and CO2 molecule. Indeed, attack of the central atom of the CoPc in the CoPc− CO2−Li1-I by a CO2 molecule leads to blocking of the CoPc backbone. Details regarding the addition of the Li atom and CO2 molecule to the CoPc−CO2−Li1-I species are discussed in the Supporting Information. On the other hand, CoPc− Li1−CO2-I is found to be the most stable species obtained from addition of a CO2 molecule to the CoPc−Li backbone (Figure 2). Before analyzing the effect of increasing the number of the Li atoms and of CO2 molecules, we find it convenient to compare CoPc−Li−CO2 to the related CoPc− Li−X (X = H2O, N2 and CH4) systems. This is because of the possible role of a variety of molecules such as H2O, N2, and CH4 present in the environment surrounding the battery. The presence of these compounds may lead to different products along with complex side reactions in the cathode of the Libased batteries. Hence, we studied the adsorption energy of H2O, N2, and CH4 on the CoPc−Li1-I backbone and compared to the corresponding value for CO2. Incorporation of H2O, N2, and CH4 molecules to the CoPc−Li1-I backbone leads to two (CoPc−Li1−H2O-I and CoPc−Li1−H2O-II), five (CoPc−Li 1 −N 2 -I, CoPc−Li 1 −N 2 -II, CoPc−Li 1 −N 2 -III, CoPc−Li1−N2-IV and CoPc−Li1−N2-V), and four (CoPc− Li1−CH4-I, CoPc−Li1−CH4-II, CoPc−Li1−CH4-III and CoPc−Li1−CH4-IV) stable species with optimized geometries reported in the Supporting Information. The calculated results show that adsorption energy of H2O (−1.39 eV) in the CoPc− Li system is much larger than that of CO2 (−0.75 eV), meaning that H2O needs to be removed from the CoPc-based battery environment or, alternatively, to change the central atom. Calculations for equivalent systems composed of the FePc, LiPc, and H2Pc backbones show that the this will not solve the problem caused by the presence of water. In contrast, the adsorption energy of N2 (−0.51 eV) and of CH4 (−0.40 eV) is sufficiently smaller than that of CO2 (−0.75 eV), so as to not represent a problem. Next, we consider the addition of a second CO2 molecule to the CoPc−Li1−CO2-I species. From the various possibilities systematically investigated, the CoPc−Li1−(CO2)2-I in Figure

Figure 3. Face (left), top (center), and side (right) views of the CoPc−Li1−(CO2)2-I most stable structure. Again, the I symbol is just to show the most stable species.

again accompanied by a linear geometry of the adsorbed CO2 molecules and weak interactions of the noncovalent character. In addition, the fact that C2O4 is not spontaneously formed by addition of CO2 molecules to CoPc−Li1−CO2-I strongly suggests that changing the CO2 concentration on the CoPc−Li backbone is not an effective way to form the Li-promoted oxalate on the CoPc. The other possibility is to start from systems with an increased number of Li atoms. Hence, we considered adding the first CO2 molecule to the CoPc−Li2-I backbone with the most stable CoPc−Li2−CO2-I structure reported in Figure 4.

Figure 4. Face (left), top (center), and side (right) views of the most stable structure of CoPc−Li2-I and CoPc−Li2−CO2-I. Again, the I symbol is just to show the most stable species.

The adsorption energy of the CO2 molecule relative to that of the CoPc−Li2-I backbone is −1.24 eV, a rather larger value accompanied by noticeable activation of the CO2 molecule featuring a bent structure. Nevertheless, we have added a second, initially linear, CO2 molecule to the CoPc−Li2−CO2-I system and searched for possible local minima of the resulting CoPc−Li2−(CO2)2 system. Interestingly, a Li2C2O4 subunit is now formed with different isomers (see the Supporting Information) with the most stable one reported in Figure 5. The adsorption energy per CO2 molecule of −2.16 eV along with a direct interaction between the central Co atom of the CoPc and the O atom of the Li2C2O4 subunit with the Li atoms interacting only with the N atoms of the CoPc subunit are the main features of the CoPc−Li2C2O4-I species. An energy analysis reveals decomposition of CoPc−Li2C2O4-I into D


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In eq 4, n and F stand for the electron number and Faraday constant, respectively, and ΔG° is as given in eq 3. Our results for CoPc−Li1-I and CoPc−Li2-I predict an average voltage of Li-intercalation of 1.40 and 1.51 V, respectively. These values are smaller than average voltage of Li-intercalation reported for LiCoO2 (3.75 V).50 For other similar compounds with general LiMO2 (M = Ti, V, Mn, Co, Ni, Cu, Zn, and Al) stoichiometry, the same authors reported values in the 2.14− 4.70 V range,50 whereas for LiMPO4 (M = Fe, Mn, Co, and Ni) compounds, higher intercalation voltage values have been reported in the 3.47−5.07 V range;51 a similar situation is found for the LiMSiO4 (M = Fe, Mn, Co, and Ni) family with values from 4.87 to 5.18 V.51 Therefore, the average Liintercalation voltage values confirm a rather good ability of the CoPc backbone to use in the cathode of the Li-based batteries. 3.4. Electrolyte Influence on the Adsorption and Decomposition Energies and Li-Intercalation Voltage. One of the significant parts of the Li-based batteries is the electrolyte. Therefore, a meaningful study requires addressing the electrolyte influence on the adsorption and decomposition energies and on the Li-intercalation voltage. To this end, dimethyl sulfoxide (DMSO) has been chosen as the aprotic electrolyte and its effect simulated by means of the solvation model based on density (SMD)52 as implemented in the NWChem code.48 This is a refinement of the well-known polarizable continuum model.53 The present calculations with the SMD model show that the presence of DMSO increases adsorption energy of the CO2 molecules on the CoPc (the CoPc−Li2C2O4-I species) from −2.16 eV (without electrolyte) to −3.73 eV (with electrolyte). Moreover, the presence of DMSO affects the decomposition of the CoPc−Li2C2O4-I species into CoPc and the Li2C2O4-I subunit, which becomes 0.64 eV, significantly smaller than the corresponding value of 1.16 eV obtained when the presence of DMSO is not taken into account. Clearly, the presence of the DMSO electrolyte facilitates detachment of the Li2C2O4-I subunit from the CoPc. Finally, we have calculated the Li-intercalation voltage for the CoPc−Li1-I and CoPc−Li2-I species in the presence of the DMSO to provide a clear vision of the influence of the electrolyte. The calculations show that the presence of the DMSO electrolyte leads to Li-intercalation voltages for CoPc− Li1-I and CoPc−Li2-I species of 1.94 and 2.04 V, respectively; both are larger than the corresponding values, where the electrolyte is not taken into account, 1.40 and 1.51 V, respectively. These results strongly suggest that the presence of the aprotic electrolyte (DMSO) has important and constructive effects on the adsorption and decomposition energies and Li-intercalation voltage. It should be pointed out that in the context of a Li-based battery, an ideal electrolyte should have specific properties such as a wide enough electrochemical window, chemical stability, a high ionic conductivity with extremely low electronic conductivity, low flammability, and low cost.12 Therefore, the selection of a suitable electrolyte is one of important challenges. For instance, room-temperature ionic liquids have emerged as promising replacement electrolytes because of their unique properties as illustrated by molecular dynamics simulations.54,55 Nevertheless, the exact evaluation of the electrolyte influence on the performance of the CoPc nanoflake in a Li-based battery requires a comprehensive and complex study, which is out of scope of the present work. 3.5. ET Mechanism and Related Properties. To be able to ascertain whether ET in the CoPc−Li2C2O4-I species

Figure 5. Face (left), top (center), and side (right) views of the most stable structure of CoPc−Li2C2O4 arising from the optimization of initial CoPc−Li2−(CO2)2 species. Note that the resulting Li2C2O4 subunit is clearly visible. The I symbol denotes the most stable species.

CoPc and the Li2C2O4-I subunit is 1.16 eV, indicating that simple separation of the Li2C2O4-I species from the CoPc subunit while not thermodynamically favored does not represent a high energy cost. To further investigate the role of the central Co atom in the electronic and structural properties of the CoPc−Li2−(CO2)2 system, we have replaced the central Co atom with the Fe atom. Interestingly, this change does not affect the formation of the Li2C2O4 subunit, although it is accompanied by a larger thermodynamic stability of the FePc−Li2C2O4 system relative to the CoPc-containing equivalent system. Note that removing the Li2C2O4 subunit is also 0.40 eV smaller for the CoPc backbone. To inspect the effect of higher concentrations of Li and CO2 on the formation of the Li2C2O4 subunit on the CoPc backbone, we investigated in some details the optimized geometries of the CoPc−Li4−(CO2)4 system (doublet PES). The CoPc−(Li2C2O4)2-I most stable species clearly displays the formation of two Li2C2O4 subunits (Figure 6), and analysis

Figure 6. Face (left), top (center) and side (right) views of the most stable structure of CoPc−(Li2C2O4)2-I arising from the optimization of initial CoPc−Li4−(CO2)4 species. Note that the resulting Li2C2O4 dimer is clearly visible. The I symbol denotes the most stable species.

of results reveals that detachment of the Li2C2O4 dimer from the CoPc−(Li2C2O4)2-I system requires 1.57 eV, a value only 0.41 eV higher than the one required to remove single Li2C2O4 subunit from the CoPc−Li2C2O4-I system. 3.3. Li-Intercalation Voltage. The Li-intercalation voltage is a meaningful criterion to evaluate the properties of a cathode material in Li-based batteries. To obtain the average Liintercalation voltage (V̅ ), using eq 4, we focus on the CoPc− Li1-I and CoPc−Li2-I species, which include the electrostatic interaction between [Li]+ and [CoPc]− ions, which have already been used by us in a previous study.44 V̅ =

−ΔG 0 nF

(4) E


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The Journal of Physical Chemistry C proceeds through space or through bond mechanism, we investigated the properties of the intramolecular interaction between the CoPc and Li2C2O4 subunits and the corresponding induced change of atomic charges. From the analysis of electron density (ρ(r)) and of its Laplacian (∇2ρ(r)), it appears that all intramolecular interactions between the CoPc and Li2C2O4 subunits have noncovalent character, indicating a through-space mechanism. This is confirmed by calculated Bader (AIM) and Weinhold (NBO) charges for the Li2C2O4 subunit in a rather large range of Co−O3 bond length (1.50− 4.30 Å). The plot in Figure 7 shows that along this quite large

Figure 8. Schematic picture of sequential CoPc−Li2−(CO2)2 → IM1 → CoPc−Li2C2O4-I and CoPc−Li2−(CO2)2 → IM2 → CoPc− Li2C2O4-I pathways and of the concerted CoPc−Li2−(CO2)2 → CoPc−Li2C2O4-I one for the overall transfer of two electrons.

Figure 7. Variation of the Bader and NBO charge of the oxalate moiety in the CoPc−Li2C2O4-I species as a function of the Co−O distance.

distance range, there is a very small variation of the total AIM charge of the Li2C2O4 subunit from 0.05e to 0.01e. A similar trend is observed for the NBO charges. These results confirm that the mechanism of ET in the CoPc−Li2C2O4-I species is through space. The analyses discussed so far indicate that addition of two linear CO2 molecules to the CoPc−Li2-I species triggers the formation of the Li2C2O4 subunit, which is accompanied by the transfer of two electrons from the CoPc to the initially linear CO2 molecules evolving to the oxalate moiety. We focus now on the possible intermediates along the ET process, leading to the formation of the Li2C2O4 subunit following the scheme in Figure 8. Identification of these intermediates provides clear vision about possible pathways of the ET process. Because Li2C2O4 formation in CoPc−Li2C2O4-I involves a doublet PES, spin density can be chosen as a convenient descriptor (see Figure S4 in the Supporting Information). Note also that the CDFT method along with constraint of spin density on either the CoPc subunit or on the CO2 molecules has been used to obtain the geometry of the initial state structure (CoPc−Li2−(CO2)2) and that of the IM1 and IM2 intermediates involved in the ET process shown in Figure 8, with their CDFT optimized structures reported in Figure 9. We recall that the final product is the CoPc− Li2C2O4-I already discussed (Figure 5). The analysis of the spin density of the CFDT-optimized CoPc−Li2−(CO2)2 structure shows that unpaired electron density is localized on the CoPc backbone, which together with the linear geometry of the CO2 molecules in this species confirms the lack of ET process from the CoPc subunit to the CO2 molecules as shown in Figures 8 and S4 in the Supporting Information. Therefore, CoPc−Li2−(CO2)2 provides a mean-

ingful initial state to investigate whether ET involves sequential or concerted pathways as illustrated in Figure 8. The CDFT method leads to IM1 and IM2 as possible intermediates for the first ET process from the CoPc subunit to the CO2 molecules. In the IM1, the unpaired electron density distributes on both CO2 molecules, which leads to the formation of a [C2O4]− subunit; note that here the oxalate moiety has a single negative charge. In contrast, in the IM2 intermediate, one of the CO2 molecules plays the role of electron acceptor, leading to formation of a [CO2]− subunit. Subsequently, both the IM1 and the IM2 convert to the CoPc−Li2C2O4-I species through a second ET process from the CoPc subunit to the CO2 molecules. Here, the Li2C2O4 subunit is electrically neutral, and the spin density of the CoPc subunit is similar to that of isolated CoPc, which confirms the second ET process. In the concerted pathway, the transfer of two electrons from the CoPc subunit of the CoPc−Li2−(CO2)2 species to the CO2 molecules leads directly to the formation of the CoPc− Li2C2O4-I final product. The CDFT approach allows for a quantitative identification of the geometries in the involved ET process but does not provide information on which of the two possible mechanism is preferred. We address this selectivity issue between sequential and concerted pathways through thermodynamic and kinetic properties of ET. To this end, we focus on the ΔG°, Hab, and λ characteristic parameters of the ET process for all channels in the sequential and concerted pathways with values reported in Table 1. Note that these parameters are necessary to estimate the kab rate constant involved in the ET as given in eq 3 and the corresponding Gibbs free energy (ΔG⧧) process as given in eq 5 F


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small kab values of 9.95 × 10−42 and 1.06 × 10−15 s−1, respectively. On the other hand, large kab values of 4.85 × 1012 and 6.89 × 104 s−1 are obtained for the two steps in the sequential mechanism through IM2. Therefore, this intermediate with one unpaired electron on one of the CO2 molecules appears to play a significant role in ET reaction. Note, however, that the rather simple model used in the present work is aimed at providing just preliminary information regarding how ET takes place in the cathode of the Li-based battery. It is expected that the present model will provide a first step toward more sophisticated models, which will provide a more quantitative description of the ET in the cathode of the Li-based battery. Further refinements require studying the influence of temperature, cycling performance, possible byproducts, decomposition potential of the Li2C2O4 product, battery reversibility, replacement of Li+ ions with other metal ions, such as Na+ and K+, too high concentration of Li and CO2, among others. Yet, the present model is thought to capture a large part of the underlying physics.

4. CONCLUSIONS Hereby, we investigated a possible effective solution to improve the performance of the Li−CO2 battery involving the replacement of the Li2CO3 product by Li2C2O4. By means of computational DFT-based methods, we studied the systematic addition of Li atoms and CO2 molecules to a cobalt phthalocyanine (CoPc) nanoflake and investigated the properties of such molecular catalyst for the cathode of the Li− CO2 battery. Other MPcs involving Fe, Ni, Cu, Zn, or Mg have been considered, but the properties of the CoPc in terms of binding energy of Li and the central metal atom and electron affinity of the CoPc were superior. The present study shows that CoPc−CO2−Li systems are not suitable candidates for the formation of the C2O4 subunit on the CoPc through increasing concentration of the Li atom and CO2 molecules. However, adding the Li atom and CO2 molecule to the CoPc−Li−CO2 system (2:2 ratio for Li and CO2) leads to the formation of a stable Li2C2O4 product with high adsorption energy of −2.16 eV. The average calculated Li-intercalation voltage for the CoPc nanoflake is smaller than that of LiCoO2, LiMO2 (M = Ti, V, Mn, Co, Ni, Cu, Zn, and Al), LiMPO4 (M = Fe, Mn, Co, and Ni), and LiMSiO4 (M = Fe, Mn, Co, and Ni) families of compounds, indicating that the CoPc material is appropriate for use in the cathode of the Li-based batteries. The presence of the electrolyte (DMSO) has important and constructive influence on the adsorption and decomposition energies and Li-intercalation voltage. The study of the intramolecular interaction between the CoPc and Li2C2O4 subunits and of the variation of atomic charges confirm that the mechanism of ET in the CoPc−Li2− (CO2)2 system is through space. The ET mechanism involved in the formation of the neutral Li2C2O4 product involves a sequential pathway with an intermediate with one unpaired electron on one of the CO2 molecules resulting from the first ET.

Figure 9. Face (left), top (center), and side (right) views of the geometries of the initial and intermediates in Figure 8 optimized by means of CDFT with the constraint of spin density on either CoPc (CoPc−Li2−(CO2)2) or the CO2 molecules (IM1 and IM2 species). Note that the geometry of the final product in Figure 8 is given in Figure 5.

Table 1. Parameters Involved in the ET Processes through the Channels given in Figure 8a pathways

ΔG°

λ

ΔG⧧

Hab

sequential-IM1

−0.53 −2.31 −0.28 −2.56 −2.84

2.16 0.30 0.88 1.04 0.68

0.31 3.37 0.10 0.56 1.72

0.09 0.15 0.12 0.10 0.07

sequential-IM2 concerted

kab 5.95 9.95 4.85 6.89 1.06

× × × × ×

108 10−42 1012 104 10−15

See definitions in eqs 3 and 5. Note that the “a” and “b” subindexes correspond to the diabatic states in the Marcus theory. a

ΔG‡ =

(λ + ΔG°)2 4λ

(5)

The ΔG° values reported in Table 1 confirm that ET reaction in all channels of the sequential and concerted pathways is a thermodynamically spontaneous process. The smallest and the largest ΔG° values correspond to the CoPc− Li2−(CO2)2 → IM2 (−0.28 eV) and the CoPc−Li2−(CO2)2 → CoPc−Li2C2O4-I (−2.84 eV) channels, respectively. To obtain information about the kinetic properties, we have estimated the reaction barrier ΔG⧧ of the ET reaction for all channels. The results reflect that the sequential pathway through IM1 and the concerted pathway involve high ΔG⧧ values of 3.37 and 1.72 eV, respectively. In contrast, the two ΔG⧧ values in the sequential pathway through IM2 (0.10 and 0.56 eV) are significantly smaller, strongly suggesting that the mechanism through IM2 is kinetically preferred. This is supported by the kab values for all channels in Table 1. The sequential through IM1 and concerted pathways have very

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b06395. G


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Additional information for the CoPc−CO2−Li−CO2, CoPc−CO 2 −Li−(CO 2 ) 2 , CoPc−CO 2 −(Li) 2 , and CoPc−(Li)2−(CO2)2 systems; optimized geometries for the CoPc−CO 2 −Li−CO 2 , CoPc−CO 2 −Li− (CO2)2, CoPc−(Li)2−(CO2)2, and XPc−Li−Y (X = Co, Fe, Li, and H; Y = H2O, CH4, and N2) systems; spin density distribution in possible sequential and concerted pathways for transfer of two electrons from CoPc to the CO 2 molecules; and adsorption energy per CO 2 molecule at the PBE0-D3 level and adsorption energies (Eads) of different species at the PBE0-D3/cc-pVDZ// PBE0-D3/6-31G(d) level (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.N.). *E-mail: [email protected] (F.I.). ORCID

Francesc Illas: 0000-0003-2104-6123 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.N. is grateful to the Institute for Advanced Studies in Basic Sciences for financial support through research grant no. G2016IASBS32604. F.I. acknowledges support from the Spanish Ministry of Economy and Competitiveness (MINECO) and Fondo Europeo de Desarrollo Regional (FEDER) grant CTQ2015-64618-R and, in part, from Generalitat de Catalunya grants 2017SGR13 and XRQTC and from the 2015 ICREA Academia Award for Excellence in University Research.

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Assessing the Performance of Cobalt-Phthalocyanine Nanoflakes as

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